Research On Three-level Three-phase Load Unbalance Correction Device

Nowadays,with the development of the power electronics industry,power quality problems such as three-phase unbalance,reactive power,and harmonics in power systems are becoming more and more serious.Based on the three-phase four-wire diode clamp type three-level topology,this paper designs an active compensation device for three-phase load unbalance,which can accurately compensate unbalanced load and is suitable for complex industrial site,such as grid distortion and large load harmonic content.Based on the working principle of the topology,this paper constructs the circuit model and the corresponding mathematical model.On this basis,the core technical problems such as grid synchronization algorithm,current command extraction algorithm and current tracking control algorithm are deeply studied.The second-order generalized integration algorithm is used to realize grid synchronization and current command extraction,and the PI+PR+ repetition control algorithm is used to to achieve output current tracking control.In this paper,the relationship between the DC side voltage and the compensation current is analyzed by using mathematical model.The voltage control strategy is proposed to compensate various currents,especially the negative sequence current.The waveform quality of the output current and the compensation effect are improved under the premise of ensuring the DC side voltage is relatively stable.Aiming at the problem that the LCL filter is prone to resonance in the device,this paper deeply studies the root cause of resonance and the suppression strategy used at present,and proposes an integrated damping control method,combined with active damping and passive damping to realize the suppression of output current resonance.At the same time,based on zero-crossing analysis,this paper proposes a resonance detection algorithm,which responds in time when the grid-connected current resonates to improve the stability of the device and provides a basis for the integrated resonance avoidance control strategy.In this paper,the simulation system is built based on PLECS software,and the effectiveness of the proposed algorithm is verified in the simulation platform.At the same time,the threephase load unbalance compensation device with a capacity of 100 kVar is designed,and the485 and Wi-Fi communication modules are integrated,and the device is monitored by using a LCD or an Android system App.The effectiveness of the proposed algorithm is verified by compensating for various unbalanced loads.